1. Field of the Invention
The present invention relates to a stabilized injection locking system for lasers in which the light from a master laser is phase modulated before it is injected into a slave laser.
2. Description of the Prior Art
Injection locking of two continuous wave (CW) lasers is known in the art. Examples of such systems are disclosed in xe2x80x9cServo-Loop Based on Heterodyne Interferometry for Injection Locking of CW Nd: YAG Lasersxe2x80x9d, by Lesage, et al., Optics Communications, Vol. 115, pages 291-296, Mar. 15, 1995; xe2x80x9cOptical Injection Locking and Phase Lock Loop Combined Systemsxe2x80x9d, by Ramos, et al., Optics Letters, Vol. 19, No. 1, Jan. 1, 1994, pages 4-6; xe2x80x9cHigh-Performance Homodyne Optical Injection Phase-Lock Loop Using Wide-Linewidth Semiconductor Lasersxe2x80x9d, by Bordonalli, et al., IEEE Photonics Technology, Vol. 8, No. 9, pages 1217-1219, September 1996; and xe2x80x9cInjection Locking of Argon-Ion Lasersxe2x80x9d, by Man, et al., Optics Letters, Vol. 9, No. 8, August 1984, pages 333-334, hereby incorporated by reference. In general, injection locking of CW lasers is a process in which a fraction of the output of one of the CW lasers (i.e. a master laser) is injected into a second CW laser (i.e. slave laser) such that the injected signal drives the slave laser""s lasing mode. In most cases, the spectrum of the slave laser output will contain two terms: a strong term near the free-running slave laser frequency and a weak term at the frequency of the injected signal. If the frequency of the injected signal is close enough to that of the free running slave laser, the weak input captures the slave laser output. The slave laser output spectrum becomes monochromatic at the injected frequency, and the two lasers are injection locked. The range of frequency differences over which this process occurs is called the locking range.
Unfortunately, the locking range is relatively small compared to typical laser frequency drift rates. As such, two injection locked lasers without frequency compensation will typically lose lock due to frequency drift in a relatively short period of time. As such, such lasers are known to be provided with some form of frequency drift control in order to provide long term stability of the injection locking. In such systems, as the frequency difference between the master laser and the free-running slave laser varies within the locking range, the relative phase between the two optical signals varies. As such, the frequency drift control schemes must measure the phase difference either directly or indirectly since there is no way to measure the frequency difference directly if the lasers are locked. An example of a scheme which uses a direct approach is disclosed in xe2x80x9cHIGH PERFORMANCE HOMODYNE OPTICAL INJECTION PHASE LOCK LOOP USING WIDE-LINE WIDTH SEMI-CONDUCTOR LASERSxe2x80x9d supra. In the direct approach, an optical homodyne detector is used to measure the phase difference between the two laser signals. Unfortunately, there are two disadvantages with such direct approach schemes. These disadvantages relate to the complexity and the required stability of the homodyne optics and the requirement that the phase shift xcfx86=0 within the cavity be enforced based on a measurement of the relative optical phase outside the cavity.
In order to avoid these disadvantages, indirect phase measurement methods have been developed. One of the more common indirect phase measurement injection locking schemes is disclosed in xe2x80x9cLASER PHASE AND FREQUENCY STABILIZATION USING AN OPTICAL RESONATORxe2x80x9d, by Drever, et al. Applied Physics Vol. 31, pages 97-105, hereby incorporated by reference. In this approach, a laser is stabilized by applying a phase dither to the output of the laser, coupling the phase modulated light into a passive optical resonator (the reference cavity) and measuring the amplitude dither at the modulation frequency in the light reflected from the reference cavity. This amplitude dither is used as a control signal since it is bipolar and is zero when the laser frequency is exactly resonant with the reference cavity. The frequency of the phase dither is chosen to be large enough such that the phase modulation (PM) sidebands are far enough away from the optical carrier that they are reflected by the cavity instead of being transmitted. When this locking technique is used to stabilize injection locking, the slave laser acts as a reference cavity and the basic stabilization mechanism is unchanged because the PM sidebands do not couple into the slave cavity and do not affect the injection locking.
Briefly, the present invention relates to an injection locking system for lasers. In particular, a signal from a master laser is phase modulated and injected into a slave laser. The optical phase difference xcfx86 between the master and slave lasers is maintained at zero by way of a phase locked loop. By maintaining the phase difference xcfx86 at zero, the frequency drift is compensated by maintaining the frequency difference at a fixed point within the locking range.